To gain circuit surface and/or to be able to use different manufacturing technologies in a same electronic device, it is known to stack microelectronic circuits, this type of integration being commonly called “3D micropackaging”.
Usually, the circuits are electrically and mechanically connected by means of interposers comprising a silicon, glass, or graphite layer thoroughly crossed by metal electrodes in contact with electric areas or tracks of the circuits, thus allowing their communication and/or their electric power supply. In the state of the art, it is desired to have interposers combining the following features for the electrode support layer: a good electric isolation to avoid shorting the electrodes and/or to avoid leakage currents, a good heat conductivity to dissipate the heat, and a positive thermal expansion coefficient as close as possible to the thermal expansion coefficient of the circuits connected to the interposer, which circuits are usually made of materials having a positive thermal expansion coefficient, such as for example semiconductor materials and metals. Such design constraints are for example described in US 2008/0277776 and US 2004/0195669.
While a silicon or glass interposer has a low cost and a good heat dispersion, as the silicon volume decreases due to the thinning of the interposer, for example, for miniaturization reasons, the metal volume of the through connections proportionally becomes more significant, which may cause a significant heating of the interposer.
Further, a silicon interposer has a high rigidity which makes its use difficult in certain applications, for example in the context of electronic skins. Finally, the metals forming the connections have a very high Young's modulus, which embrittles the silicon layer where they are formed.
Further, the through metal electrodes are formed by etching holes in a silicon or glass layer previously formed by a deposition technique, and then by filling these holes with a metal. Now, the forming of through holes having a form factor of good quality, particularly holes having a constant cross-section across the entire thickness of a silicon or glass layer, requires using complex and expensive etching technologies.
To overcome these problems, the use of an interposer made of an organic material, particular of an organochlorine material, such as polychlorinated biphenyls, has been provided. Indeed, polychlorinated biphenyls have the advantage of being flexible and a polychlorinated biphenyl layer provided with regularly-distributed through holes may be simply formed in a single step by means of low-cost manufacturing techniques, particularly by silk-screening.
However, polychlorinated biphenyls are poor heat conductors and cannot by themselves dissipate significant heat flows, often reaching values greater than 300 W/cm2, usually encountered in microelectronic circuits. Heat thus builds up in this type of interposer, causing a thermal shock which damages both the interposer and the circuits to which it is connected. Further, polychlorinated biphenyls have a strong dielectric permittivity for high frequencies, which disturbs the operation of circuits operating at high frequencies.
Further, whatever the type of interposer of the state of the art, a large difference between the positive thermal expansion coefficient of the layer having the electrodes formed therein and the positive thermal expansion coefficient of the metal used for the electrodes can generally be observed. When it is submitted to strong thermal variations, the different portions of the interposer thus expand or contract differently, thus embrittling the assembly.
Finally, the circuits connected through an interposer are also submitted to different contractions or expansions, particularly because they have different expansion coefficients, and/or because they are submitted to different temperature variations, which here again embrittles the assembly.